The present invention relates to a digital AM demodulator, and, more particularly, to a high-frequency digital AM demodulator adapted for demodulating TV signals in PAL-NTSC-SECAM formats, which can be integrated in a digital video processing unit. The digital AM demodulator starts with a medium-frequency sampled signal, such as a signal originating from a tuner.
To demodulate a TV signal starting from a medium-frequency sampled signal originating from a tuner, for example, an analog demodulator is positioned downstream from the tuner. The output is a signal that is demodulated in an audio demodulator to obtain an audio signal, and a video signal that is split into a luminance signal and a chrominance signal.
We now consider a known AM demodulator, a block diagram of which is shown in FIG. 1. The demodulator is formed by an input filter 1 and by a low-pass filter 2 arranged in a cascade configuration. The input filter 1 is fed with an input signal and the output signal therefrom is fed to a multiplier 3, which in turn receives a local carrier signal.
Assuming, therefore, that a local carrier designated by the term xe2x80x9ccarrierxe2x80x9d is available, whose value is
carrier=2 cos(xcfx89at+"psgr"),
and according to trigonometric addition and subtraction formulas             cos      ⁡              (        α        )              +          cos      ⁡              (        β        )              =      cos    ⁢                            (                      α            +            β                    )                2            ·      cos        ⁢                  (                  α          +          β                )            2      
the carrier (carrier) and the received signal x(t) produce the demodulation of A(t) according to the following relations:
input x=A(t)xc2x7cos(xcfx89it+xcfx86)
carrier=2xc2x7cos(xcfx89at+"psgr")
yt=inputxc2x7carrier
yt=2xc2x7A(t)xc2x7cos(xcfx89it+xcfx86)xc2x7cos(xcfx89at+"psgr")=A(t)xc2x7cos[(xcfx89i+xcfx89a)t+(xcfx86+"psgr")]+cos[(xcfx89ixe2x88x92xcfx89a)t+(xcfx86xe2x88x92"psgr")]
After multiplication, one obtains a frequency composed of two components, as shown in FIG. 3. One is modulated around the frequency (xcfx89ixe2x88x92xcfx89a), the other one is modulated around the frequency (xcfx89i+xcfx89a). If the higher-frequency component is removed with a low-pass filter and the condition xcfx89i=xcfx89a is set, one obtains:
y=A(t)xc2x7cos[(xcfx89ixe2x88x92xcfx89a)t+(xcfx86xe2x88x92"psgr")=A(t)xc2x7cos(xcfx86xe2x88x92"psgr")t=A(t)xc2x7k(t)
wi=xcfx89a
|k|xe2x89xa61
Since the coefficient k has a modulus of 1 or less, and to also provide the demodulated channel with the maximum energy level (SNR=max, where SNR is the signal/noise ratio), it is necessary to set:
xcfx89a=xcfx89i= greater than y=A(t)xc2x7cos(xcfx86xe2x88x92"psgr")
xcfx86="psgr"= greater than y=A(t)
Using the same reasoning and with reference to FIGS. 3 and 4, it can be easily demonstrated that it is possible to perform baseband demodulation of the transposed input channel or of its exact symmetrical counterpart by acting on the position of the frequency of the local carrier. If fc=fi the baseband spectrum is the input spectrum, and if fc=fh; then a symmetrical spectrum is obtained.
The above is a direct demodulation. We now describe an AM demodulation achieved in two steps, i.e., with the aid of two carriers, designated by carrier1 and carrier2. With reference to the diagram of FIG. 2, the reference numeral 1 designates, as in the preceding case, an input filter, whereas the reference numeral 2 designates the low-pass filter and 3 is the multiplier to which the first carrier, carrier1, is fed. The diagram of FIG. 2 provides for a second multiplier 4 to which the second carrier, carrier2, is fed and an additional low-pass filter 5 is arranged downstream of the multiplier 4.
In this case, one has the following relations:
input=A(t)xc2x7cos(xcfx89it+xcfx86)
carrier1=2xc2x7cos(xcfx89at+"psgr")
carrier2=2xc2x7cos(xcfx89bt+xcfx86)
(xcfx89a+xcfx89b)=xcfx89i
ya=inputxc2x7carrier1
xe2x80x83ya=2xc2x7A(t)xc2x7cos(xcfx89it+xcfx86)xc2x7cos(xcfx89at+y)=A(t){cos[(xcfx89i+xcfx89a)t+(xcfx86+"psgr")]+cos[(xcfx89ixe2x88x92xcfx89a)t+(xcfx86xe2x88x92"psgr")]}
After eliminating the high-frequency component (xcfx89i+xcfx89a) one obtains:
yb=A(t)xc2x7cos[(xcfx89ixe2x88x92xcfx89a)t+(xcfx86+"psgr")]
After the first multiplication in the first multiplier 3, the spectrum A(t) is modulated around the intermediate frequency xcfx89int=(xcfx89ixe2x88x92xcfx89a), with the initial phase fint=(xcfx86xe2x88x92"psgr"). The spectrum has been shifted to a lower frequency. With the second stage, shown in FIG. 2, one obtains:
yc=ybxc2x7carrier2
yc=2xc2x7A(t)xc2x7cos(wintt+xcfx86int)xc2x7cos(xcfx89bt+xcfx86)
yc=A(t){cos[(wint+xcfx89b)t+(xcfx86int+xcfx86)]+cos[(xcfx89intxe2x88x92xcfx89b)t+(xcfx86intxe2x88x92xcfx86)]}
and after the low-pass filter 5
y=A(t)xc2x7cos[(xcfx89intxe2x88x92xcfx89b)t+(xcfx86intxe2x88x92xcfx86)]
The conditions under which the input signal A(t) is correctly baseband-demodulated are as follows:
xcfx89i=(xcfx89a+xcfx89b)xe2x80x83xe2x80x83condition 1
xcfx86=("psgr"+xcfx86)xe2x80x83xe2x80x83condition 2
It can be noted that both the frequency xcfx89i and the phase xcfx86 can be distributed at will between the two local carriers, carrier, and carrier2. Accordingly, the following problems arise in the design of a demodulator, particularly of the digital type.
Since at the output of a tuner the phase xcfx89 is not known in advance and the medium-frequency spectrum has a frequency shift which can vary by a few hundred kHz with respect to its nominal value (for a video tuner the band is 33-38.9 MHZ+/xe2x88x92100 kHz), the need for a PLL is evident for analog recovery signals to extract xcfx89i in phase with the received signal f. For medium-high frequencies (f greater than 20 MHz), digital PLLs are typically more burdensome than analog equivalents for an equal performance, and are therefore rarely used.
Another drawback is noted with discrete-time systems where t=nTs, with T equal to the sampling period. In this case, the frequency spectrum is periodic, with a period 2p, and this can cause an unwanted aliasing effect during demodulation, as shown in FIG. 5b. 
In FIG. 5a, the carrier lies at the vertex B of the input spectrum and demodulation occurs correctly. In FIG. 5b, the carrier lies in the upper part of the spectrum, at the vertex A. In this case, the first two repeats of the sampled input spectrum are demodulated, distorting the band signal. This effect is not observed in the case of analog demodulation (see FIGS. 3 and 4) and forces a two-step demodulation, as shown in FIG. 5c. 
In this case, the first demodulation is meant to shift the input spectrum to a lower frequency and to shift the second demodulation to a higher frequency. With the second demodulation, the channel is brought to baseband correctly without aliasing errors, but this nearly doubles the complexity of the architecture, which uses at least two multipliers and three filters, as shown in FIG. 2.
An object of the present invention is to provide a digital AM demodulator in which recovery of the frequency and phase of the carrier is performed without resorting to a PLL.
A further object of the present invention is to provide a digital AM demodulator which can be easily integrated in digital video decoders.
Another object of the present invention is to provide a digital AM demodulator in which the complexity of the circuit and the occupied area are reduced with respect to known approaches.
Yet another object of the present invention is to provide a digital AM demodulator which is highly reliable, relatively easy to manufacture and is done so at competitive costs.
These objects and others which will become apparent hereinafter are achieved by a digital AM demodulator, particularly for demodulating a signal originating from a tuner, comprising means for generating a first carrier which is not correlated with the input signal to be demodulated, a first multiplier for multiplying the first carrier by the input signal to be demodulated, and a plurality of filters arranged upstream and downstream of the first multiplier and suitable to eliminate unwanted spectral emissions.
The digital AM demodulator further comprises means for detecting the phase shift between the frequency of the input signal to be demodulated and a local carrier, and means for correlating the first carrier with the input signal. The first carrier and the local carrier are mutually correlated, and the local carrier is not correlated with the input signal to be demodulated.